Various heat transfer techniques heretofore known in the art include a sublimation heat transfer technique in which a sublimable dye is carried on a substrate sheet such as paper as a recording medium to form a heat transfer sheet. The heat transfer sheet is then overlaid on an image-receiving material, which is dyeable with the sublimable dye, such as a polyester woven fabric. A heat energy is finally applied to the assembly from the back side of the heat transfer sheet in a patterned form to transfer the sublimable dye onto the image-receiving material.
In recent years, it has also been proposed to make use of the above-mentioned sublimation type of heat transfer technique to form various full-color images on paper or plastic films. In this case, the heating means used is a printer's thermal head which can transfer a number of color dots of three or four colors onto the image-receiving material by very quick heating, thereby reproducing a full-color image representation of a manuscript with the multicolor dots.
The thus formed image is very clear and excels in transparency due to the coloring material used being a dye, so that it can be improved in terms of the reproducibility of neutral tints and gray scale. Thus, it is possible to form a high-quality image equivalent to an image achieved by conventional offset or gravure printing and comparable to a full-color photographic image.
The most serious problems with the above-mentioned heat transfer technique, however, arise in connection with the color density, storability and resistance to discoloration/fading of the formed image.
In other words, a heat energy for fast recording should be applied within as short a time as possible, say, the fraction of a second. Within such a short time, however, both the sublimable dye and the image-receiving material cannot fully be heated, thus failing to make an image representation of sufficient density.
For that reason, a sublimable dye having an improved sublimability has been developed accommodate to such fast recording. However, since the sublimable dye of an improved sublimability has generally a low molecular weight, it is likely to migrate or bleed through the image-receiving material subjected to heat transfer, as time passes by. This leads to a storability problem that the formed image may become disfigured or blurred, or otherwise contaminate surrounding articles.
In order to avoid such a problem, it has been proposed to use a sublimable dye having a relatively high molecular weight. With this, however, it has been impossible to form any image of satisfactory density, since its rate of sublimation is insufficient for such a fast recording technique as mentioned above.
Another problem is that because of being formed by the dye, the resulting image is generally so inferior in light resistance to a pigmented image that it fades or discolors prematurely upon exposure to direct sunlight. This light resistance problem may be solved to some extent by adding UV absorbers or antioxidants to a dye-receiving layer of the image-receiving material.
However, the discoloration/fading problem arises not only by direct sunlight but also by other light, e.g., indoor light, or even under conditions not directly exposed to light, e.g., in albums, cases and books. These indoor or in-the-dark discoloration/fading problems can never be solved by using general UV absorbers or antioxidants.
It is therefore an object of this invention to provide a heat transfer sheet used with the heat transfer technique making use of a sublimable dye, which can give a clear image of not only sufficient density but also improved fastness properties, esp., improved storability and resistance to discoloration/fading.